Alkali metal phosphate coating method for metals and article produced thereby



ALKALI METAL PHOSPHATE COATING METHOD FOR METALS AND ARTICLE PRODUCED I THEREBY Application October 20, 1954 Serial No. 463,605

8 Claims. (Cl. 148-616) No Drawing.

Ti This invention relates to articles having an improved protective surface coating thereon of the type which results from contacting the metal surface with an aqueous acidic solution, and to the method for producing such articles. I

The formation of phosphate coatings on the surfaces (iffabricated metallic articles for corrosion resistance and as a'base for organic finishes, such as paint and varnish, is well known and widely employed. Phosphate solutions are of two basic types, namely the type in which the metallic ion of the phosphate solution become a part of the coating and which is generally called coating phosphates and the type in which the metallic ion in the solution does not become a part of the coating, the metal ion in the coating coming from the surface being coated, generally called non-coating phosphates. The coating phosphates are exemplified by zinc and manganese phosphates while the non-coating phosphates are typified by the alkali metal phosphates such as sodium, potassium and ammonium phosphates. U.S. Patent No. 1,791,715 teaches that the corrosion resistance of a phosphate coating of the coating phosphate type may be increased by treating the coating with a chromic acid solution containihg not more than 3 grams/liter CrO or 5 grams/ liter of an alkali metal chromate. For coatings produced from coating phosphate solutions in the presence of the N or N0 radicals, U.S. Patents Nos. 2,067,214 and 2,067,016 disclose the desirability of rinsing the coatings with chromic acid solutions containing up to about 5 grams of Q0, per liter.

U.S. Patent No. 2,609,308 teaches the rinsing of phosphate coatings resulting from non-coating phosphate solutions with dilute solutions of chromic acid, mixtures of chromic acid and phosphoric acid or heavy metal dichromates. The dilute solutions of U.S. Patent No. 2,609,308 and the conventional commercial practice of tre ating phosphate coatings resulting from either the coatingor non-coating type of solution produce an extremely ,and light-weight layer of chromic acid on the coating on the order of about 0.1-0.2 milligram of Q0, per square foot of surface, or less. More than about 0.2 mg sq. ft. of CrO has not been customarily used because larger quantities produce undesirable yellow stains, the rinse solution tends to dissolve away the phosphate coating or the quality of the ultimately painted surface is impaired.

In accordance with the present invention, it has now bge n found that unusually good corrosion resistance and receptivity for adhesives, paints and the like is provided in coatings resulting from treating ferrous surfaces, such as steel, with aqueous alkali metal phosphate solutions if thecoating is subsequently treated with a concentrated aqueous chromate solution which forms an unusually heavy chromate upper layer. This invention is based on the: discovery that iron phosphate coatings which result from treating a ferrous surface with an aqueous acidic alkali metal phosphate solution are relatively inert to extremely strong hexavalent chromium containing solutions 'CrO per liter.

which would quickly dissolve conventional zinc or manganese phosphate coatings, and moreover that the heavy chromate coating formed on the surface provides unexpected benefits in the form of increased corrosion resistance and improved adhesion for siccative organic coatings such as plastic coatings, paints, enamels and the like. Particularly outstanding results have been obtained in connection with lacquers used in can closures.

The superior articles of this invention are those having a ferrous surface which is coated with an iron phosphate underlayer and an overlayer of iron-chromate or ironphosphate-chromate, the overlayer having a weight in the range of about 0.60 to about 15 milligrams per square foot of CrO When the term iron phosphate is employed in this specification and in the appended claims, it is to be understood that what is referred to is the coating which results from contacting a ferrous surface with a conventional aqueous alkali metal phosphate solution under customary conditions, as disclosed for example in U.S. Patent No. 2,609,308. When the weight of CrO is below 0.6 mg./sq. ft. the increase in corrosion protection is much less than above 0.6. The higher weights of CrO deposited on the phosphate-coated surface tend to make the surface tacky. The point of tackiness varies somewhat with each type of chromate compound used in the chromatizing step. In general, a safe upper limit for zinc chromate is about 8 mg./ sq. ft., while with aluminum chromate tackiness sets in at about 2 mg./sq. ft. It is to be understood that this tackiness spoken of above is undesirable from a commercial standpoint due to the tendency for the coated metal sheets to stick together. The higher weights of CrO however, do produce better corrosion resistance and adherence of paint and lacquer coatings such that there may be instances where special handling is justified. For general use, however, the optimum weight of the chromate coating may be in the range of 1 to 4 mg./sq. ft.

As mentioned hereinabove, chromate or chromic acid rinses employed as the final step in conventional phosphate coating processes are relatively weak solutions and in commercial operations rarely exceed about 1 gram of The chromate films resulting from the use of such solutions is exceedingly thin, sometimes invisible, and usually has a weight in the range of 0.1 to about 0.2 mg./sq. ft. In contrast, the chromate films of this invention are 3 to times heavier and these coatings serve to greatly extend the field of usefulness of coatings from alkali metal phosphate solutions.

The method of this invention comprises the steps of contacting a ferrous surface with an aqueous alkali metal phosphate solution for a time sufficient to form an ad herent uniform coating thereon, and thereafter contacting such coated surface with a concentrated hexavalent chromium-containing solution for a time suflicient to form a chromate film having a weight between about 0.6 and 15 milligrams of CrO /sq. ft. of surface area and thereafter drying the coating.

The alkali metal phosphate solutions which may he sat isfactorily employed are the aqueous acidic phosphate solutions of sodium, potassium and ammonium having a pH in the range of 4.2 to 6.0. These solutions may optionally contain any of the oxidizing agents known to be suitable for the purpose of accelerating surface attack such as chlorates, bromates, nitrites, sulfites, chlorites and hydrogen peroxide. For most applications, it is desirable that an oxidizing agent be present, and the best results have been obtained when the proportion of oxidizing agent present was somewhat higher than that customarily employed, for example, 3% chlorate or its equivalent in accelerating effect.

The chromate coating may be formed by the use of aqueous solutions of the dichromates of zinc, calcium,

Patented Apr. 14, 1959 3 cadmium and aluminum in concentrated form and preferably containing about 7 to 25 grams/liter of zinc dichromate, ZnCr O or its equivalent in hexavalent chromium derived from another source. Of course, solutions containing lower concentrations than these can be utilized if a greater amount of solution is allowed to adhere to the surface to build up the desired weight of chromate on the surface. If desired, chromic acid may be employed but the dichromate form is preferred because of the smaller amount of reactivity between the dichromates and the iron phosphate coating. Zinc dichromate and aluminum dichromates are preferred over the other dichromates because they produce a more uniform film that is unaffected by changes in temperature or humidity.

The alkali metal phosphate solutions or the dichromate solutions may be applied by dipping or spraying. Chromate coatings of uniform thickness may be more easily obtained by passing the coated article between rubber rolls to remove the excess dichromate: solution if desired. Many articles are of such size and shape that passage between rubber rolls is not feasible and in this case the thickness of the chromate coating can be controlled by varying the concentration of the dichromate solution. After the article is removed from the chromate solution, no rinse should be used. The deposited chromate coating is dried in place in air.

Example 1 A phosphate-coating bath was prepared by dissolving 100 grams of monosodium phosphate and 55 grams of sodium chlorate in 8 liters of water. This solution was aged by immersing l2 5 x black plate steel panels therein and its acidity readjusted with 75% phosphoric acid or 25% NaOH so as to have a total acidity of 10 points (or ml. of N/ 10 NaOH required to neutralize a 25 ml. sample to a phenolphthalein end point). The bath was then heated to 155-l60 F. before application to the metal. Black plate steel panels were first cleaned, then given a hot water rinse of 5 to 10 seconds to remove the cleaner solution and heat the panels, then the panels were sprayed for either 5 or 10 seconds with the above phosphate bath, and finally rinsed with hot water. The panels so treated received a uniform, adherent iron phosphate coating weighing from about 25 to about 70 mg./sq. ft. The pH of the bath varied between 4.0 and 5.2 during the treatment of the panels.

The resulting phosphate-coated steel sheets were conducted directly from the final hot water rinse to a spray booth where they were sprayed for 5 to 10 seconds with a solution of zinc chromate containing 2.9% CrO and having a pH of 4.0. The panels were then rolled with a roller to remove excess zinc chromate solution and dried in an air oven. The resulting panels were found to have acquired a visible chromate coating whose weight was determined within one-half hour of application by rinsing sample panels with Water, acidifying the rinse water with sulfuric acid and titrating it with ferrous sulfate using ferrous orthophenanthroline complex as an end-point indicator. The weight of zinc chromate, as CrO deposited 0n the phosphate coating was found to be from 1.8 to 2.9 mg./sq. ft. of surface.

Control panels were prepared in the same manner by processing the same type of black plate steel panels in the above phosphate solution but the phosphate-coated panels were given a conventional chromic acid rinse in a solution containing 0.05% CrO The panels of this example, including the controls, were subjected to salt spray and humidity tests. The test panels of this example were definitely superior in salt spray resistance and equal or slightly superior to the controls in the humidity test. Some of the test panels and controls were additionally tested by a Cooker test in which the panels were first coated on one side with sufficient of a sanitary lacquer to form a dried coating of 0.03 mm.. the lacquer dried and the panel then wet with water and suspended in a can containing cream-style corn of the standard grade sold in the neighborhood grocery store. The can is then sealed and cooked for 60 or 70 minutes or even for as much as two to four hours. The can is then cooled in water, allowed to stand for 24 hours and then opened, the panels removed, washed and dried. The adhesion of the lacquer to' the chromatized phosphated surfaces and to the surfaces of the control panels is then determined by pressing Scotch tape against the lacquered surface, removing the Scotch" tape with a jerk and observing the amount of lacquer removed. While this test is not a precise quantitative test, it revealed wide differences in lacquer adherence between the control panels and the panels of this example. The adherence of the lacquer to the control panels was very poor whereas that of the heavily chromatized surfaces was sufiiciently good to enable the use of the inexpensive phosphatized-chromatized-lacquered black plate as a substitute for lacquered tin plate in the canning of foodstuffs and the like.

Example 2 To demonstrate that both the phosphate and chromate coatings are essential for good corrosion resistance and chlorate and having a pH of about 5.2 for 10 seconds at F. In some cases, the phosphate coating'was omitted while in all cases, the panels were rinsed in zinc chromate solutions. In these cases, the panels were not roller leveled so that somewhat weaker CrO solu-" tions produced heavier chromate coatings. The data are as follows:

Percent 010;, Cooker Sample Phosphate CrO; Rinse. mg.lsq. Rating,

Used in Rinse pH tt. 1 hr.

In the corn Cooker tests a rating of below 0.2 is

taken as good. It will be noted that the panel having the phosphate coating only (#28) had a very poor cooker rating and that having only the conventional 0.05% CrO rinse alone (#30) was also totally'inetfecmum chromate coating of 0.6 mg./sq. ft. shows a fair to good Cooker rating.

1 mg./sq. ft., the Cooker rating was zero. In fact, samples 35 and 38 had a zero rating after a two-hour Cooker test whereas sample #30 had a two hour Cooker rating of 6.8. It is clear that both the phosphate and heavy chromate coatings are ineffective alone whereas when a phosphate coating is combined with a chromate coating weighing at least 0.6 mg./sq. ft., the metal successfully passes the drastic Cooker test showing that it acceptable for use in the production of food containers.

Example 3 i In this example, chromic acid, aluminum dichromate and calcium dichromate were substituted for the-zinc chromate used in the chromate rinses of the previous examples. The phosphate solution utilized was a 10' point monosodium phosphate bath having a pH of aboutv 5.2. The processing time and temperaturewas 10 secends at 160 F. It was found that wherever the weight" Starting with sample #37, wherever the weight of percent soluble Cr0 was above of hexavalent chromium overcoating on the phosphate coating was above 0.6 mg./sq. ft., irrespective of the source compound, the treated metal passed the cream corn Cooker test. In the above series using aluminum dichromate, rinses containing 0.19 to 0.65% CrO at a pH from 3.2 to 3.6 produced chromate coatings weighing from 1.5 to 10.8 mg./sq. ft. and showing a Cooker rating of 0.0 to 0.11 after one hour and 0.01 to 0.36 after two hours. It was found, however, that wherever the weight of aluminum dichromate was above 2 mg./ sq. ft. the surface became tacky although results in the Cooker test were very good. The calcium dichromate coatings were found to be somewhat affected by temperature and humidity, the coating changing from a uniform film to an irregular crystal line type coating which was poor in appearance but still an excellent base for organic siccative finishes. In the case of chromic acid, the adhesion of sanitary lacquer in the cream corn Cooker test was good although the higher concentrations of this compound required seemed to dissolve the phosphate coating to a considerable extent. The results indicated that zinc and aluminum dichromates were to be preferred and that zinc chromate was the best, easiest to control and the least expensive.

Example 4 Certain additives were found to be of assistance in obtaining a heavier weight of CrO on the surface with a rinse of a given CrO content. For example, 0.2% of a non-ionic detergent, known as Igepal CA and believed to be an alkyl phenol polyoxyethylene type condensate, was added to a point (pH 5.2) monosodium phosphate bath accelerated with 3% chlorate. It was found that without the detergent, the total weight of coating obtained in 10 seconds at 160 F. with a rinse solution (zinc chromate) containing 0.2 to 0.25% CrO was about 37 mg./sq. ft. With .2% of the detergent about the same weight of coating was obtained in only five seconds at 160 F. Likewise, the weight of chromate coating deposited by a 0.2% CrO zinc chromate solution was 1.5 to 3.4 mg. as compared to 1.2 to 1.8 mg. without the detergent. In all cases, the adhesion of the sanitary lacquer to the surfaces was essentially unchanged by the presence of the detergent.

In other cases, small amounts of sodium acid fluoride, Versene ethylene diamine tetraacetic acid in the form of an alkali metal salt, bentonite clay, and a kerosene emulsion seemed to increase the total weight of coating and the weight of the chromate coating as compared to controls made without these additives. In all cases, the Cooker" test results were good. These additives may find use in coating diflicult-to-coat or slightly dirty metal or to facilitate coatings in high speed automatic coating machines.

What is claimed is:

1. An article having a ferrous surface having an iron phosphate coating thereon and an overlayer containing at least 0.6 mg./sq. ft. and not more than about mg./sq. ft. of a hexavalent chromium compound calculated as CrO 2. An article having a ferrous surface which is coated with an iron phosphate underlayer, an overlayer containing from 0.6 to 15 mg./sq. ft. of a hexavalent chromium compound calculated as CIO,, and, adhered thereover, an organic siccative finish material.

3. An article of black plate steel having on its surface an iron phosphate underlayer and an overlayer containing from 0.6 to 15 mg./sq. ft. of a hexavalent chromium compound calculated as G0,.

4. A sheet of low carbon steel suitable for use in the production of containers and can closures having a black plate finish, a uniform adherent underlayer of iron phosphate, a layer of hexavalent chromium compound over said phosphate coating and weighing from 1 to 4 mg./sq. ft. calculated as CrO and a top layer of sanitary lacquer.

5. The method of treating ferrous surfaces to increase their resistance to corrosion and improve the adherence thereto of organic siccative finishes which comprises providing an iron phosphate coating thereon, rinsing said phosphate-coated surface with a concentrated solution of a hexavalent chromium compound, and drying said rinse on said surface, controlling the concentration of said rinse solution and the conditions of said rinsing operation so as to deposit an overlayer of hexavalent chromium compound on said surface weighing at least 0.6 mg./sq. ft. and not more than about 15 mg./sq. ft. calculated as CrO 6. The method of treating ferrous surfaces to increase their resistance to corrosion and improve the adherence thereto of organic siccative organic finishes which comprises treating said surface with a solution of a non-coating phosphate, rinsing said surface with a concentrated solution of a hexavalent chromium compound, and drying said rinsing solution on said phosphate-coated surface, controlling the concentration of said rinse solution and the conditions of said rinsing operation so as to deposit an overlayer of hexavalent chromium compound on said surace weighing at least 0.6 mg./sq. ft. calculated as Cr 7. The method of treating ferrous surfaces to increase their resistance to corrosion and improve the adherence thereto of organic siccative organic finishes which comprises treating said surface with a solution of a noncoating phosphate so as to deposit thereon a uniform, adherent iron phosphate coating, rinsing said phosphatecoated surface with a concentrated solution of a hexavalent chromium compound selected from the class consisting of the dichromates of zinc, calcium, cadmium and aluminum, and drying said rinsing solution on said phosphate-coated surface, controlling the concentration of said rinse solution and the conditions of said rinsing operation so as to deposit an overlayer of hexavalent chromium compound on said surface Weighing between 0.6 and 15 mg./sq. ft. calculated as 01:0

8. The method of treating the surface of black plate steel to increase its resistance to corrosion and improve the adherence thereto of siccative organic finishes which comprises treating said surface with a solution of a noncoating phosphate so as to deposit thereon a uniform, adherent iron phosphate coating, rinsing said phosphate coating with a concentrated solution of a hexavalent chromium compound selected from the class consisting of the dichromates of zinc, calcium, cadmium and aluminum, and drying said rinsing solution on said phosphate-coated surface, controlling the concentration of said rinse solution and the conditions of said rinsing operation so as to deposit on said surface an overlayer of hexavalent chromium compound weighing from 1 to 4 mg./sq. ft.

References Cited in the file of this patent UNITED STATES PATENTS 2,067,216 Thompson et a1. Ian. 12, 1937 2,314,565 Thompson Mar. 23, 1943 2,403,426 Douty et a1. July 2, 1946 2,665,231 Amundsen Ian. 5, 1954 

5. THE METHOD OF TREATING FERROUS SURFACES TO INCREASE THEIR RESISTANCE TO CORROSION AND IMPROVE THE AHERENCE THERETO OF ORGANIC SICCATIVE FINISHES WHICH COMPRISES PROVIDING AN IRON PHOSPHATE COATING THEREON, RINSING SAID PHOSPHATE-COATED SURFACE WITH A CONCENTRATED SOLUTION OF A HEXAVALENT CHROMIUM COMPOND, AND DRYING SAID RINSE ON SAID SURFACE, CONTROLLING THE CONCENTRATION OF SAID RINSE SOLUTION AND THE CONDITIONS OF SAID RINSING OPERATION SO AS TO DEPOSIT AN OVERLAYER OF HEXAVALENT CHROMIUM COMPOUND ON SAID SURFACE WEIGHING AT LEAST 0.6 MG./SQ. FT. AND NOT MORE THAN ABOUT 15 MG./SQ. FT. CALCULATED AS CRO3. 